Assessment of physical stability of an antibody drug conjugate by higher order structure analysis: impact of thiol- maleimide chemistry

Understanding the Effects of Site-Specific Light Chain Conjugation on Antibody Structure Using Hydrogen Exchange-Mass Spectrometry (HX-MS)

Antibody drug conjugates (ADCs) represent one of the fastest growing classes of cancer therapeutics. Drug incorporation through site-specific conjugation in ADCs leads to uniform drug load and distribution. These site-specific modifications may have an impact on ADC quality attributes including protein higher order structure (HOS), which might impact safety and efficacy. In this study, we conducted a side-by-side comparison between the conjugated and unconjugated mAb. In the ADC, the linker-pyrrolobenzodiazepine was site specifically conjugated to an engineered unpaired C215 residue within the Fab domain of the light chain. Differential scanning calorimetry (DSC) and differential scanning fluorimetry (DSF) indicated a decrease in thermal stability for the CH2 transition of the ADC. Size exclusion chromatography (SEC) analysis showed that conjugation of the mAb resulted in earlier aggregation onset and increased aggregation propensity after 4 weeks at 40 °C. Differential hydrogen-exchange mass spectrometry (HX-MS) indicated that upon conjugation, light chain residues 150-155 and 197-204, close to the conjugation site, showed significantly faster HX kinetics, suggesting an increase in backbone flexibility within this region, while heavy chain residues 32-44 exhibited significantly slower kinetics, suggesting distal stabilization of the mAb backbone.

Cetuximab-based PROteolysis targeting chimera for effectual downregulation of NSCLC with varied EGFR mutations

PROteolysis Targeting Chimeras (PROTACs) showed tremendous therapeutic potential in degrading several oncoproteins including undruggable proteins. PROTACs are bifunctional molecules where one-part binds to target protein while the other end recruits protein degradation machinery. With the unveiling advancements in the field of PROTACs, we explored a combinatorial approach by developing antibody-based PROTAC (ABTAC) which may effectively degrade one of the key oncoprotein driving proliferation and progression of cancer - Epidermal growth factor receptor (EGFR). The objective of current research was to synthesize and characterize an EGFR degrading ABTAC for the treatment of non-small cell lung cancer (NSCLC). Cetuximab and pomalidomide (E3 ligase recruiting ligand) were conjugated using lysine conjugation and copper free azide-alkyne cycloaddition (CuAAC) click chemistry. Analytical characterization using reverse-phase liquid chromatography and mass spectrometry suggested conjugation of five E3-ligase inhibitor molecules/antibody. Nearly 10-30 folds reduction in IC50 was observed with ABTAC in HCC827 (EGFR sensitive) and H1650 (EGFR resistant) cells compared to cetuximab. Multicellular 3D spheroid assay strongly suggested that ABTAC induced significant apoptosis and also inhibited cell proliferation compared to control and antibody alone. Circular dichroism and surface plasmon resonance (SPR) confirmed minor alterations in the structure and receptor binding efficacy of the antibody post-conjugation.

Analyzing protein conjugation reactions for antibody-drug conjugate synthesis using polarized excitation emission matrix spectroscopy

Antibody-drug conjugates (ADCs) are promising anticancer therapeutics, which offer important advantages compared to more classical therapies. There are a variety of ADC critical quality attributes (CQAs) such as the protein structure, aggregation, and drug-to-antibody ratio (DAR), which all impact on potency, stability, and toxicity. Production processes can destabilize antibodies via a variety of physical and chemical stresses, and or by increased aggregation after conjugation of hydrophobic drugs. Thus, a proper control strategy for handling, production, and storage is necessary to maintain CQA levels, which requires the use of in-process quality measurements to first identify, then understand, and control the variables which adversely affect ADC CQAs during manufacturing. Here, we show how polarized excitation emission matrix (pEEM) spectroscopy, a sensitive, nondestructive, and potentially fast technique, can be used for rapidly assessing aggregation and DAR in a single measurement. pEEM provides several sources of information for protein analysis: Rayleigh scatter for identifying aggregate/particle formation and fluorescence emission to assess chemical and structural changes induced by attachment of a linker and/or a small molecule drug payload. Here, we used a nontoxic ADC mimic (monoclonal antibody with linker molecule) to demonstrate efficacy of the measurement method. Emission changes caused via light absorption by the attached linker, allowed us to predict DAR with good accuracy using fluorescence signal from the final purified products (6% relative error of prediction [REP]) and also from unpurified alkylation intermediates (11% REP). pEEM changes could also be correlated with size (hydrodynamic radius, R_h ) and aggregate content parameters obtained from dynamic light scattering and size exclusion chromatography (SEC). For the starting material and purified product samples, pEEM correlated better with R_h (R²  = 0.99, 6% REP) than SEC determined aggregate content (18% REP). Combining both fluorescence and light scatter signals also enabled in-process size quantification (6% REP). Overall, combining polarized measurements with EEM and Rayleigh scatter provides a single measurement, multi-attribute test method for ADC manufacturing.

Antibody-Incorporated Nanomedicines for Cancer Therapy

Antibody-based cancer therapy, one of the most significant therapeutic strategies, has achieved considerable success and progress over the past decades. Nevertheless, obstacles including limited tumor penetration, short circulation half-lives, undesired immunogenicity, and off-target side effects remain to be overcome for the antibody-based cancer treatment. Owing to the rapid development of nanotechnology, antibody-containing nanomedicines that have been extensively explored to overcome these obstacles have already demonstrated enhanced anticancer efficacy and clinical translation potential. This review intends to offer an overview of the advancements of antibody-incorporated nanoparticulate systems in cancer treatment, together with the nontrivial challenges faced by these next-generation nanomedicines. Diverse strategies of antibody immobilization, formats of antibodies, types of cancer-associated antigens, and anticancer mechanisms of antibody-containing nanomedicines are provided and discussed in this review, with an emphasis on the latest applications. The current limitations and future research directions on antibody-containing nanomedicines are also discussed from different perspectives to provide new insights into the construction of anticancer nanomedicines.

Structural characterization of a monomethylauristatin-E based ADC that contains 8 drugs conjugated at interchain cysteine residues

Antibody-drug conjugates (ADCs) with a drug-to-antibody ratio (DAR) of 8 are attractive as therapeutic anti-cancer agents due to the higher levels of cytotoxic payload delivered to tumors. Biophysical characterization of a DAR 8 ADC fully conjugated at all interchain cysteine residues was carried out to determine if IgG1 interchain disulfide reduction and conjugation led to structural perturbations that impacted product stability. Comparisons between the DAR 8 ADC and the unconjugated parent antibody identified minor tertiary and quaternary structural changes localized to the C_L, C_H1, and C_H2 domains and C_H2-C_H3 domain interface. Stability studies of the DAR 8 ADC indicated that the structural changes had minimal impacts to product stability as demonstrated by low levels of fragmentation and aggregation under nominal storage and temperature stress stability conditions. Additionally, no detectable higher order structural changes were observed by CD or DSC in the DAR 8 ADC after 3 months at (25 °C) stability conditions. The structural and stability results support the developability of DAR 8 ADCs fully conjugated to interchain cysteines residues with an optimized and clinically relevant second generation monomethylauristatin-E (MMAE) drug-linker.

Aggregation of protein therapeutics enhances their immunogenicity: causes and mitigation strategies

Protein aggregation in biotherapeutics has been identified to increase immunogenicity, leading to immune-mediated adverse effects, such as severe allergic responses including anaphylaxis. The induction of anti-drug antibodies (ADAs) moreover enhances drug clearance rates, and can directly block therapeutic function. In this review, identified immune activation mechanisms triggered by protein aggregates are discussed, as well as physicochemical properties of aggregates, such as size and shape, which contribute to immunogenicity. Furthermore, factors which contribute to protein stability and aggregation are considered. Lastly, with these factors in mind, we encourage an innovative and multidisciplinary approach with regard to further research in the field, with the overall aim to avoid immunogenic aggregation in future drug development.

Site-specific glycan-conjugated NISTmAb antibody drug conjugate mimetics: synthesis, characterization, and utility

Antibody drug conjugates (ADCs) represent a rapidly growing modality for the treatment of numerous oncology indications. The complexity of analytical characterization method development is increased due to the potential for synthetic intermediates and process-related impurities. In addition, the cytotoxicity of such materials provides an additional challenge with regard to handling products and/or sharing materials with analytical collaborators and/or vendors for technology development. Herein, we have utilized a site-specific chemoenzymatic glycoconjugation strategy for preparing ADC mimetics composed of the NIST monoclonal antibody (NISTmAb) conjugated to non-cytotoxic payloads representing both small molecules and peptides. The materials were exhaustively characterized with high-resolution mass spectrometry-based approaches to demonstrate the utility of each analytical method for confirming the conjugation fidelity as well as deep characterization of low-abundance synthetic intermediates and impurities arising from payload raw material heterogeneity. These materials therefore represent a widely available test metric to develop novel ADC analytical methods as well as a platform to discuss best practices for extensive characterization.

Development and preclinical evaluation of cixutumumab drug conjugates in a model of insulin growth factor receptor I (IGF-1R) positive cancer

Overexpression of insulin growth factor receptor type 1 (IGF-1R) is observed in many cancers. Antibody drug conjugates (ADCs) with PEGylated maytansine (PEG₆-DM1) show promise in vitro. We developed PEG₆-DM1 ADCs with low and high drug to antibody ratios (DAR) using an anti-IGF-1R antibody cixutumumab (IMC-A12). Conjugates with low (cixutumumab-PEG₆-DM1-Low) and high (cixutumumab-PEG₆-DM1-High) DAR as 3.4 and 7.2, respectively, were generated. QC was performed by UV spectrophotometry, HPLC, bioanalyzer, and biolayer-interferometry. We compared the in vitro binding and internalization rates of the ADCs in IGF-1R-positive MCF-7/Her18 cells. We radiolabeled the ADCs with ¹¹¹In and used microSPECT/CT imaging and ex vivo biodistribution to understand their in vivo behavior in MCF-7/Her18 xenograft mice. The therapeutic potential of the ADC was studied in vitro and in mouse xenograft. Internalization rates of all ADCs was high and increased over 48 h and EC₅₀ was in the low nanomolar range. MicroSPECT/CT imaging and ex vivo biodistribution showed significantly lower tumor uptake of ¹¹¹In-cixutumumab-PEG₆-DM1-High compared to ¹¹¹In-cixutumumab-PEG₆-DM1-Low and ¹¹¹In-cixutumumab. Cixutumumab-PEG₆-DM1-Low significantly prolonged the survival of mice bearing MCF-7/Her18 xenograft compared with cixutumumab, cixutumumab-PEG₆-DM1-High, or the PBS control group. Cixutumumab-PEG₆-DM1-Low ADC was more effective. The study highlights the potential utility of cixutumumab-ADCs as theranostics against IGF-1R positive cancers.

Evaluation of hydrophobic-interaction chromatography resins for purification of antibody-drug conjugates using a mimetic model with adjustable hydrophobicity

Antibody drug conjugates are cytotoxic pharmaceuticals, designed to destroy malignant cells. A cytotoxic molecule is attached to an antibody that binds specific to a cancer‐cell surface. Given the high toxicity of the drugs, strict safety standards have to be kept. For this reason, an antibody drug conjugates model was developed with fluorescein 5‐isothiocyanate as the nontoxic payload surrogate. Due to the similar hydrophobicity, this model is used to establish a suitable purification process and characterization method for antibody drug conjugates. Because of the pH dependent solubility of fluorescein, the hydrophobicity of conjugates can be modulated by the pH value. Based on the complex heterogeneity and hydrophobicity of the conjugates a chromatographic purification is challenging. Hydrophobic interaction chromatography is used for analytical as well as for preparative separation. Because of the increased hydrophobicity of the conjugates compared to native antibody, hydrophobic interaction chromatography often suffer from resolution and recovery problems. Conjugates were separated differing on the number of payloads attached to the antibody. For this matter, the drug–antibody ratio is determined and used as a quantitative term. The conjugates are purified at high recoveries and resolution by step gradients using suitable resins, allowing the separation of the target drug–antibody ratio.

Effect of Linker-Drug Properties and Conjugation Site on the Physical Stability of ADCs

The physical stability of antibody drug conjugates is dictated by the properties of the antibody, linker-drug, and conjugation site. Two linker-drugs were chosen that are different in terms of hydrophobicity and polar surface area to evaluate the effect of linker-drug properties on antibody-drug conjugate (ADC) behavior. Site-specific and non-site-specific conjugation was used to investigate the role of conjugation site in conformational and colloidal stability. Finally, 2 antibodies were selected to determine if the observed results were antibody-specific. The conformational stability is affected, with the highest degree of destabilization observed when conjugation results in the removal of interchain disulfide bonds. Although conformational destabilization occurred in the domain in which conjugation occurred and domains distinct from the conjugation site, no correlation could be drawn between linker-drug properties and conformational stability. Evaluation of aggregation by size exclusion HPLC confirmed a relationship between linker-drug hydrophobicity and aggregation propensity under thermal stress in all ADCs tested. The extent of aggregation was far greater in the conjugates generated with a more hydrophobic antibody, illustrating that the properties of both the antibody and linker-drug contribute to aggregation. These studies emphasize that the distinct properties of the molecule as a whole warrant a case-by-case evaluation of each ADC.

Improved Physical Stability of an Antibody-Drug Conjugate Using Host-Guest Chemistry

Antibody-drug conjugates (ADCs) are an emerging class of biopharmaceutical products for oncology, with the cytotoxic pyrrolobenzodiazepine (PBD) family of "warheads" well-established in the clinic. While PBDs offer high potency, they are also characterized by their hydrophobicity, which can make formulation of the ADC challenging. Several approaches have been investigated to improve the physicochemical properties of PBD-containing ADCs, and herein a supramolecular approach was explored using cucurbit[8]uril (CB[8]). The ability of CB[8] to simultaneously encapsulate two guests was exploited to incorporate a 12-mer polyethylene glycol harboring a methyl viologen moiety at one terminus (MV-PEG₁₂), together with a PBD harboring an indole moiety at the C2' position (SG3811). This formulation approach successfully introduced a hydrophilic PEG to mask the hydrophobicity of SG3811, improving the physical stability of the ADC while avoiding any loss of potency related to chemical modification.

Antibody Conjugates-Recent Advances and Future Innovations

Monoclonal antibodies have evolved from research tools to powerful therapeutics in the past 30 years. Clinical success rates of antibodies have exceeded expectations, resulting in heavy investment in biologics discovery and development in addition to traditional small molecules across the industry. However, protein therapeutics cannot drug targets intracellularly and are limited to soluble and cell-surface antigens. Tremendous strides have been made in antibody discovery, protein engineering, formulation, and delivery devices. These advances continue to push the boundaries of biologics to enable antibody conjugates to take advantage of the target specificity and long half-life from an antibody, while delivering highly potent small molecule drugs. While the "magic bullet" concept produced the first wave of antibody conjugates, these entities were met with limited clinical success. This review summarizes the advances and challenges in the field to date with emphasis on antibody conjugation, linker-payload chemistry, novel payload classes, absorption, distribution, metabolism, and excretion (ADME), and product developability. We discuss lessons learned in the development of oncology antibody conjugates and look towards future innovations enabling other therapeutic indications.

Distinctive Low-Resolution Structural Features of Dimers of Antibody-Drug Conjugates and Parent Antibody Determined by Small-Angle X-ray Scattering

Structural features of lysine-conjugated antibody-drug conjugate (ADC) from humanized IgG1 were studied by small-angle X-ray scattering (SAXS). As the physicochemical properties of the cytotoxic drug (payload) and linker may impact the conformational and colloidal stabilities of the conjugated monoclonal antibody (mAb), it is essential to characterize how the conjugation may affect the overall higher order structure and therefore the physical stability and integrity of the ADCs upon storage conditions. Here, the ADC monomer and aggregates generated upon thermal stress were analyzed by high performance liquid chromatography coupled to SAXS with a particular focus on the fraction of dimers (3-10% depending on the storage conditions at 25 and 40 °C). In addition to average parameters such as radius of gyration, molecular weight, and maximal end-to-end distance, the structural information obtained from SAXS patterns were visualized as a low-resolution average envelope of both monomers and dimers (implementation of two methods: ab initio reconstruction and modeling Fab and Fc as rigid bodies with a flexible hinge). We showed that the monomer envelope of the ADC was similar to the corresponding (nonconjugated) parent monoclonal antibody (mAb). ADC dimers appeared more compact and less polydisperse than the dimers of mAb, which was also confirmed by atomic force microscopy. The generated envelopes of the mAb dimers suggest elongated structures with one or few inter-mAb contacts at the outermost region of Fab or Fc domains. The structural features of ADC dimers are independent of the tested pH buffering system (pH 5.0/acetate and pH 6.0/histidine with or without NaCl) and characterized by multiple, tighter contacts between the Fab and Fc domains and distortion of the monomer native shape. Results from the SAXS structural study show in the present case that conjugation has favored innermost inter-ADC contacts in the dimer, which differ from the inter-mAb ones. In general, it is likely that many parameters affect inter-ADC association, including the chemical nature of linkers and drugs, degree of conjugation, conjugation sites, etc. Making a qualitative difference between mAb and ADC dimers as a function of these parameters can help point to the presence of tight associations that must be abolished in protein drug formulations.

Biophysical Methods for Characterization of Antibody-Drug Conjugates

Antibody-drug conjugates (ADC) are made up of three components: (1) a mAb specific to cells of choice, (2) a small molecule with desired end goal, and (3) a linker to covalently link drug molecule to the antibody. Bringing together the mAb, drug molecule, and the linker results in the formation of an immunoconjugate designed to selectively deliver the drug molecule to a cell of interest. Synergic effects of the mAb and drug molecule lead to destroying the target tumor cells while leaving the normal cells unharmed. However, the development of ADCs is associated with challenges due to the heterogeneity of the ADC molecules created from the conjugation process. Addition of the linker and drug moieties during processing as well as the hydrophobicity of the drug itself can lead to structural changes that may affect the stability and functional profile of the conjugated molecule. Furthermore, linkers site of attachment plays a major role in determining the conformational and colloidal properties of the ADCs. In this chapter, several characterization methods are introduced to determine the biophysical characteristics of the ADC. Protocols, data analysis as well as notes for circular dichroism, intrinsic fluorescence, ANS fluorescence, differential scanning calorimetry, and dynamic scanning fluorimetry are outlined in detail.

Modular Assembly of Multimodal Imaging Agents through an Inverse Electron Demand Diels-Alder Reaction

The combination of two imaging probes on the same biomolecule gives access to targeted bimodal imaging agents that can provide more accurate diagnosis, complementary information, or that may be used in different applications, such as nuclear imaging and fluorescence guided surgery. In this study, we demonstrate that dichlorotetrazine, a small, commercially available compound, can be used as a modular platform to easily assemble various imaging probes. Doubly labeled tetrazines can then be conjugated to a protein through a biorthogonal IEDDA reaction. A series of difunctionalized tetrazine compounds containing various chelating agents and fluorescent dyes was synthesized. As a proof of concept, one of these bimodal probes was conjugated to trastuzumab, previously modified with a constrained alkyne group, and the resulting dual-labeled antibody was evaluated in a mouse model, bearing a HER2-positive tumor. A significant uptake into tumor tissues was observed in vivo, by both SPECT-CT and fluorescence imaging, and confirmed ex vivo in biodistribution studies.

Tubulin Tyrosine Ligase-Mediated Modification of Proteins

Tubulin tyrosine ligase (TTL) catalyzes the addition of tyrosine derivatives to the C-terminal carboxylic acid of proteins. The enzyme binds to a 14-amino acid recognition sequence, termed Tub-tag, and allows for the introduction of tyrosine derivatives that carry a unique chemical handle. These handles enable subsequent bioorthogonal reactions with a great variety of probes or effector molecules. Clearly, this two-step chemoenzymatic approach, facilitates the site-specific functionalization of proteins. Furthermore, due to its broad substrate tolerance, tubulin tyrosine ligase also enables an enzymatic one-step modification. For example, a coumarin amino acid was utilized to generate fluorescently labeled proteins for advanced applications in imaging and diagnostics. Here we describe the modification of proteins using TTL in detail via a one-step as well as two-step procedure and highlight its practicability for applications in imaging, diagnostics, and cell biology.

Conformational Assessment of Adnectin and Adnectin-Drug Conjugate by Hydrogen/Deuterium Exchange Mass Spectrometry

Higher-order structure (HOS) characterization of therapeutic protein-drug conjugates for comprehensive assessment of conjugation-induced protein conformational changes is an important consideration in the biopharmaceutical industry to ensure proper behavior of protein therapeutics. In this study, conformational dynamics of a small therapeutic protein, adnectin 1, together with its drug conjugate were characterized by hydrogen/deuterium exchange mass spectrometry (HDX-MS) with different spatial resolutions. Top-down HDX allows detailed assessment of the residue-level deuterium content in the payload conjugation region. HDX-MS dataset revealed the ability of peptide-based payload/linker to retain deuterium in HDX experiments. Combined results from intact, top-down, and bottom-up HDX indicated no significant conformational changes of adnectin 1 upon payload conjugation. Graphical Abstract ᅟ.

Enhanced Precision of Circular Dichroism Spectral Measurements Permits Detection of Subtle Higher Order Structural Changes in Therapeutic Proteins

Protein higher order structure (HOS) is an essential quality attribute to ensure protein stability and proper biological function. Protein HOS characterization is performed during comparability assessments for product consistency as well as during forced degradation studies for structural alteration upon stress. Circular dichroism (CD) spectroscopy is a widely used technique for measuring protein HOS, but it remains difficult to assess HOS with a high degree of accuracy and precision. Moreover, once spectral changes are detected, interpreting the differences in terms of specific structural attributes is challenging. Spectral normalization by the protein concentration remains one of the largest sources of error and reduces the ability to confidently detect differences in CD spectra. This work develops a simple method to enhance the precision of the CD spectral measurements through normalization of the CD spectra by the protein concentration determined directly from the CD measurement. This method is implemented to successfully detect small CD spectral changes in multiple forced degradation studies as well as comparability assessments during biologics drug development. Furthermore, the interpretation of CD spectral changes in terms of HOS differences are provided based on orthogonal data in conjunction with structural insights gained through in silico homology modeling of the protein structure.

Influence of disulfide bond isoforms on drug conjugation sites in cysteine-linked IgG2 antibody-drug conjugates

Cysteine-linked antibody-drug conjugates (ADCs) produced from IgG2 monoclonal antibodies (mAbs) are more heterogeneous than ADCs generated from IgG1 mAbs, as IgG2 ADCs are composed of a wider distribution of molecules, typically containing 0 – 12 drug-linkers per antibody. The three disulfide isoforms (A, A/B, and B) of IgG2 antibodies confer differences in solvent accessibilities of the interchain disulfides and contribute to the structural heterogeneity of cysteine-linked ADCs. ADCs derived from either IgG2-A or IgG2-B mAbs were compared to better understand the role of disulfide isoforms on attachment sites and distribution of conjugated species. Our characterization of these ADCs demonstrated that the disulfide configuration affects the kinetics of disulfide bond reduction, but has minimal effect on the primary sites of reduction. The IgG2-A mAbs yielded ADCs with higher drug-to-antibody ratios (DARs) due to the easier reduction of its interchain disulfides. However, hinge-region cysteines were the primary conjugation sites for both IgG2-A and IgG2-B mAbs.

In-Depth Comparison of Lysine-Based Antibody-Drug Conjugates Prepared on Solid Support Versus in Solution

Antibody drug conjugates are a rapidly growing form of targeted chemotherapeutics. As companies and researchers move to develop new antibody–drug conjugate (ADC) candidates, high-throughput methods will become increasingly common. Here we use advanced characterization techniques to assess two trastuzumab-DM1 (T-DM1) ADCs; one produced using Protein A immobilization and the other produced in solution. Following determination of payload site and distribution with liquid chromatography-mass spectrometry (LC/MS), thermal stability, heat-induced aggregation, tertiary structure, and binding affinity were characterized using differential scanning calorimetry (DSC), dynamic light scattering (DLS), Raman spectroscopy, and isothermal titration calorimetry (ITC), respectively. Small differences in the thermal stability of the C_H2 domain of the antibody as well as aggregation onset temperatures were observed from DSC and DLS, respectively. However, no significant differences in secondary and tertiary structure were observed with Raman spectroscopy, or binding affinity as measured by ITC. Lysine-based ADC conjugation produces an innately heterogeneous population that can generate significant variability in the results of sensitive characterization techniques. Characterization of these ADCs indicated nominal differences in thermal stability but not in tertiary structure or binding affinity. Our results lead us to conclude that lysine-based ADCs synthesized following Protein A immobilization, common in small-scale conjugations, are highly similar to equivalent ADCs produced in larger scale, solution-based methods.

Challenges and new frontiers in analytical characterization of antibody-drug conjugates

Antibody-drug conjugates (ADCs) are a growing class of biotherapeutics in which a potent small molecule is linked to an antibody. ADCs are highly complex and structurally heterogeneous, typically containing numerous product-related species. One of the most impactful steps in ADC development is the identification of critical quality attributes to determine product characteristics that may affect safety and efficacy. However, due to the additional complexity of ADCs relative to the parent antibodies, establishing a solid understanding of the major quality attributes and determining their criticality are a major undertaking in ADC development. Here, we review the development challenges, especially for reliable detection of quality attributes, citing literature and new data from our laboratories, highlight recent improvements in major analytical techniques for ADC characterization and control, and discuss newer techniques, such as two-dimensional liquid chromatography, that have potential to be included in analytical control strategies.

Qualitative analysis of antibody–drug conjugates (ADCs): an experimental comparison of analytical techniques of cysteine-linked ADCs

Four different cysteine linked antibody-drug conjugates (ADCs) consisting of Trastuzumab-vc-MMAE were analysed with four common analytical techniques with respect to drug-to-antibody ratio (DAR) and molecular weight.

In-silico prediction of concentration-dependent viscosity curves for monoclonal antibody solutions

Early stage developability assessments of monoclonal antibody (mAb) candidates can help reduce risks and costs associated with their product development. Forecasting viscosity of highly concentrated mAb solutions is an important aspect of such developability assessments. Reliable predictions of concentration-dependent viscosity behaviors for mAb solutions in platform formulations can help screen or optimize drug candidates for flexible manufacturing and drug delivery options. Here, we present a computational method to predict concentration-dependent viscosity curves for mAbs solely from their sequence-structural attributes. This method was developed using experimental data on 16 different mAbs whose concentration-dependent viscosity curves were experimentally obtained under standardized conditions. Each concentration-dependent viscosity curve was fitted with a straight line, via logarithmic manipulations, and the values for intercept and slope were obtained. Intercept, which relates to antibody diffusivity, was found to be nearly constant. In contrast, slope, the rate of increase in solution viscosity with solute concentration, varied significantly across different mAbs, demonstrating the importance of intermolecular interactions toward viscosity. Next, several molecular descriptors for electrostatic and hydrophobic properties of the 16 mAbs derived using their full-length homology models were examined for potential correlations with the slope. An equation consisting of hydrophobic surface area of full-length antibody and charges on V_H, V_L, and hinge regions was found to be capable of predicting the concentration-dependent viscosity curves of the antibody solutions. Availability of this computational tool may facilitate material-free high-throughput screening of antibody candidates during early stages of drug discovery and development.

Localized conformational interrogation of antibody and antibody-drug conjugates by site-specific carboxyl group footprinting

Establishing and maintaining conformational integrity of monoclonal antibodies (mAbs) and antibody-drug conjugates (ADCs) during development and manufacturing is critical for ensuring their clinical efficacy. As presented here, we applied site-specific carboxyl group footprinting (CGF) for localized conformational interrogation of mAbs. The approach relies on covalent labeling that introduces glycine ethyl ester tags onto solvent-accessible side chains of protein carboxylates. Peptide mapping is used to monitor the labeling kinetics of carboxyl residues and the labeling kinetics reflects the conformation or solvent-accessibility of side chains. Our results for two case studies are shown here. The first study was aimed at defining the conformational changes of mAbs induced by deglycosylation. We found that two residues in C_H2 domain (D268 and E297) show significantly enhanced side chain accessibility upon deglycosylation. This site-specific result highlighted the advantage of monitoring the labeling kinetics at the amino acid level as opposed to the peptide level, which would result in averaging out of highly localized conformational differences. The second study was designed to assess conformational effects brought on by conjugation of mAbs with drug-linkers. All 59 monitored carboxyl residues displayed similar solvent-accessibility between the ADC and mAb under native conditions, which suggests the ADC and mAb share similar side chain conformation. The findings are well correlated and complementary with results from other assays. This work illustrated that site-specific CGF is capable of pinpointing local conformational changes in mAbs or ADCs that might arise during development and manufacturing. The methodology can be readily implemented within the industry to provide comprehensive conformational assessment of these molecules.

Antibody Aggregation: Insights from Sequence and Structure

Monoclonal antibodies (mAbs) are the fastest-growing biological therapeutics with important applications ranging from cancers, autoimmunity diseases and metabolic disorders to emerging infectious diseases. Aggregation of mAbs continues to be a major problem in their developability. Antibody aggregation could be triggered by partial unfolding of its domains, leading to monomer-monomer association followed by nucleation and growth. Although the aggregation propensities of antibodies and antibody-based proteins can be affected by the external experimental conditions, they are strongly dependent on the intrinsic antibody properties as determined by their sequences and structures. In this review, we describe how the unfolding and aggregation susceptibilities of IgG could be related to their cognate sequences and structures. The impact of antibody domain structures on thermostability and aggregation propensities, and effective strategies to reduce aggregation are discussed. Finally, the aggregation of antibody-drug conjugates (ADCs) as related to their sequence/structure, linker payload, conjugation chemistry and drug-antibody ratio (DAR) is reviewed.

Antibody-drug conjugate characterization by chromatographic and electrophoretic techniques

Due to the inherent structure complexity and component heterogeneity of antibody drug conjugates (ADCs), separation technologies play a critical role in their characterization. In this review, we focus on chromatographic and electrophoretic approaches used to characterize ADCs with respect to drug-to-antibody ratio, drug distribution and conjugation sites, free small molecule drugs, charge variants, aggregates and fragments, etc. Chromatographic techniques including reversed-phase, ion exchange, size exclusion, hydrophobic interaction, two-dimensional liquid chromatography, and gas chromatography as well as capillary electrophoretic techniques including capillary electrophoresis sodium dodecyl sulfate, capillary zone electrophoresis and capillary isoelectric focusing are reviewed for their applications in the characterization of ADCs.

Molecular perspective of antibody aggregates and their adsorption on Protein A resin

Antibody aggregate is a common issue in therapeutic antibodies, which may compromise product efficacy and cause adverse effects. Antibody aggregate level is normally controlled in bioprocessing by polishing steps after Protein A capture. This paper studied the Higher Order Structures (HOS) of antibody aggregates (dimer H1 and H2) and their adsorption on Protein A resin and thus elucidated the mechanism using Protein A capture for enhanced aggregate removal. The HOS of antibody aggregates and their complex with Protein A were characterized using HDX-MS combined with SEC-MALS, Protein Conformational Array (PCA), and molecular modeling. The aggregate size and Protein A binding ratio suggested that H2 has much more compact structure than H1. HDX-MS and PCA further revealed that H1 was formed by single Fab-Fab interaction while H2 formed by Fab-Fab and likely Fc-Fc interaction. On Protein A resin, both the molar binding ratio and the correlation between protein size and ligand distance support that each monomer can only bind one Protein A ligand, while each dimer can bind two ligands, thus resulting in stronger resin binding. Furthermore, dimer H2 binds stronger than dimer H1 due to its compact structure. By integrating biophysical analysis and molecular modeling with process development, this study revealed the antibody aggregate structures and the mechanism of aggregate removal using Protein A chromatography. It also provided a general strategy for in-depth product and process understanding in antibody and other biologics development.

Cutting-edge mass spectrometry methods for the multi-level structural characterization of antibody-drug conjugates

Antibody drug conjugates (ADCs) are highly cytotoxic drugs covalently attached via conditionally stable linkers to monoclonal antibodies (mAbs) and are among the most promising next-generation empowered biologics for cancer treatment. ADCs are more complex than naked mAbs, as the heterogeneity of the conjugates adds to the inherent microvariability of the biomolecules. The development and optimization of ADCs rely on improving their analytical and bioanalytical characterization by assessing several critical quality attributes, namely the distribution and position of the drug, the amount of naked antibody, the average drug to antibody ratio, and the residual drug-linker and related product proportions. Here brentuximab vedotin (Adcetris) and trastuzumab emtansine (Kadcyla), the first and gold-standard hinge-cysteine and lysine drug conjugates, respectively, were chosen to develop new mass spectrometry (MS) methods and to improve multiple-level structural assessment protocols.

Antibody-Drug Conjugates: Design, Formulation and Physicochemical Stability

The convergence of advanced understanding of biology with chemistry has led to a resurgence in the development of antibody-drug conjugates (ADCs), especially with two recent product approvals. Design and development of ADCs requires the synergistic combination of the monoclonal antibody, the linker and the payload. Advances in antibody science has enabled identification and generation of high affinity, highly selective, humanized or human antibodies for a given target. Novel linker technologies have been synthesized and highly potent cytotoxic drug payloads have been created. As the first generation of ADCs utilizing lysine and cysteine chemistries moves through the clinic and into commercialization, second generation ADCs involving site specific conjugation technologies are being evaluated and tested. The latter aim to be better characterized and controlled, with wider therapeutic indices as well as improved pharmacokinetic-pharmacodynamic (PK-PD) profiles. ADCs offer some interesting physicochemical properties, due to conjugation itself, and to the (often) hydrophobic payloads that must be considered during their CMC development. New analytical methodologies are required for the ADCs, supplementing those used for the antibody itself. Regulatory filings will be a combination of small molecule and biologics. The regulators have put forth some broad principles but this landscape is still evolving.

Emerging formats for next-generation antibody drug conjugates

INTRODUCTION

Antibody drug conjugates now make up a significant fraction of biopharma's oncology pipeline due to great advances in the understanding of the three key components and how they should be optimised together. With this clinical success comes innovation to produce new enabling technologies that can deliver more effective antibody-drug conjugates (ADCs) with a larger therapeutic index.

AREAS COVERED

There are many reviews that discuss the various strategies for ADCs design but the last 5 years or so have witnessed the emergence of a number of different antibody formats compete with the standard whole immunoglobulin. Using published research, patent applications and conference disclosures, the authors review the many antibody and antibody-like formats, discussing innovations in protein engineering and how these new formats impact on the conjugation strategy and ultimately the performance. The alternative chemistries that are now available offer new linkages, stability profiles, drug:antibody ratio, pharmacokinetics and efficacy. The different sizes being considered promise to address issues, such as tumour penetration, circulatory half-life and side-effects.

EXPERT OPINION

ADCs are at the beginning of the next stage in their evolution and as these newer formats are developed and examined in the clinic, we will discover if the predicted features have a clinical benefit. From the commercial activity, it is envisaged that smaller or fragment-based ADCs will expand oncological applications.

Current ADC Linker Chemistry

The list of ADCs in the clinic continues to grow, bolstered by the success of first two marketed ADCs: ADCETRIS® and Kadcyla®. Currently, there are 40 ADCs in various phases of clinical development. However, only 34 of these have published their structures. Of the 34 disclosed structures, 24 of them use a linkage to the thiol of cysteines on the monoclonal antibody. The remaining 10 candidates utilize chemistry to surface lysines of the antibody. Due to the inherent heterogeneity of conjugation to the multiple lysines or cysteines found in mAbs, significant research efforts are now being directed toward the production of discrete, homogeneous ADC products, via site-specific conjugation. These site-specific conjugations may involve genetic engineering of the mAb to introduce discrete, available cysteines or non-natural amino acids with an orthogonally-reactive functional group handle such as an aldehyde, ketone, azido, or alkynyl tag. These site-specific approaches not only increase the homogeneity of ADCs but also enable novel bio-orthogonal chemistries that utilize reactive moieties other than thiol or amine. This broadens the diversity of linkers that can be utilized which will lead to better linker design in future generations of ADCs.

Approaches to Interchain Cysteine-Linked ADC Characterization by Mass Spectrometry

Therapeutic antibody-drug conjugates (ADCs) harness the cell-killing potential of cytotoxic agents and the tumor targeting specificity of monoclonal antibodies to selectively kill tumor cells. Recent years have witnessed the development of several promising modalities that follow the same basic principles of ADC based therapies but which employ unique cytotoxic agents and conjugation strategies in order to realize therapeutic benefit. The complexity and heterogeneity of ADCs present a challenge to some of the conventional analytical methods that industry has relied upon for biologics characterization. This current review will highlight some of the more recent methodological approaches in mass spectrometry that have bridged the gap that is created when conventional analytical techniques provide an incomplete picture of ADC product quality. Specifically, we will discuss mass spectrometric approaches that preserve and/or capture information about the native structure of ADCs and provide unique insights into the higher order structure (HOS) of these therapeutic molecules.

Immunogenicity of antibody drug conjugates: bioanalytical methods and monitoring strategy for a novel therapeutic modality

Immunogenicity (the development of an adaptive immune response reactive with a therapeutic) is a well-described but unwanted facet of biotherapeutic development. There are commonly applied procedures for immunogenicity risk assessment, testing strategies, and bioanalysis. With some modifications, these can be applied to new biotherapeutic modalities. For novel therapies such as antibody-drug conjugates (ADCs), the unique structural components may contribute additional complexities to both immunologic responses and bioanalytical methods. US product inserts (USPIs) for two commercially available ADCs detail the incidence of immunogenicity; however, the body of literature on immunogenicity of ADCs is limited. We recently participated in a conference session on this topic (Annual meeting of the American Association of Pharmaceutical Scientists, held November 2013 in San Antonio, TX, USA. The meeting featured the Symposium: Immunogenicity Assessment for Novel Antibody Drug Conjugates, Nonclinical to Clinical) which prompted an effort to share our perspectives on how immunogenicity risk assessment, testing strategies, and bioanalytical methods can be adapted to reflect the complexity of ADC therapeutics.

Improving the serum stability of site-specific antibody conjugates with sulfone linkers

Current routes for synthesizing antibody–drug conjugates commonly rely on maleimide linkers to react with cysteine thiols. However, thioether exchange with metabolites and serum proteins can compromise conjugate stability and diminish in vivo efficacy. We report the application of a phenyloxadiazole sulfone linker for the preparation of trastuzumab conjugates. This sulfone linker site-specifically labeled engineered cysteine residues in THIOMABs and improved antibody conjugate stability in human plasma at sites previously shown to be labile for maleimide conjugates. Similarly, sulfone conjugation with selenocysteine in an anti-ROR1 scFv-Fc improved human plasma stability relative to maleimide conjugation. Kinetically controlled labeling of a THIOMAB containing two cysteine substitutions was also achieved, offering a strategy for producing antibody conjugates with expanded valency.

Absorption, distribution, metabolism, and excretion considerations for the development of antibody-drug conjugates

Antibody-drug conjugates (ADCs) are a class of therapeutics that are designed to deliver potent small-molecule drugs selectively to cells that express a specific target antigen while limiting systemic exposure to the drug. This is accomplished by conjugating a potent drug onto an antibody-based therapeutic with a linker that is exquisitely stable in plasma. The development of an effective ADC requires optimizing a number of design elements and an extensive understanding of absorption, distribution, metabolism/catabolism, and elimination (ADME) processes for the ADC construct. Furthermore, as ADCs are a combination of an antibody and small-molecule drug, understanding key aspects of the ADME of each individual component is needed. This review aims to provide considerations for the development of ADCs from an ADME point of view.